For the purposes of the article, a Word Document is being password protected but this can be any Office Document such as PowerPoint or Excel.
When in Word, go to File and then go to Info.
In Info, select Protect Document.
When you press on Protect Document a little drop down box will appear the second one down is Encrypt with a Password.
If you lose or forget the password, it cannot be recovered. Please keep passwords for documents in a safe place.